In an engine, such as an automobile engine, power may be transmitted from the engine crankshaft to one or more accessories using an endless polymeric belt. The accessories may include, for example, an alternator, an alternator/starter, a compressor, and a power steering device. The belt may be driven by a driving sheave connected to the engine crankshaft and drive one or more driven sheaves connected to one or more accessories. When the engine crankshaft experiences rapid acceleration or deceleration, or when any driven accessory is suddenly loaded or unloaded as a result of, for example, mechanical failure, it may be difficult to maintain tension in the power-transmitting belt so as to avoid slip between the belt and the sheaves. Accordingly, a variety of belt tensioners have been proposed for maintaining the required tension in the power-transmitting belt even during non-steady-state or transient conditions. One such tensioner is disclosed in U.S. Pat. No. 4,416,647. This reference is incorporated herein by reference.
For most operating conditions the best location for a tensioner in a belt-driven accessory drive system is in the “slack span” which is located between the crankshaft sheave, which ordinarily drives the system, and a first driven accessory sheave. Under certain operating conditions, however, this location of the tensioner is not the optimal location for maintaining tension in the belt. For example, during a rapid deceleration of the crankshaft driving sheave, such as occurs during a shift from first to second gear at wide open throttle, the crankshaft sheave behaves like a driven sheave and the accessory sheave, as a result of its rotational momentum, behaves like a driving sheave. In this condition the tension in the “slack span” of the belt will rapidly increase causing the belt to lift a belt-engaging pulley of the tensioner. Lifting of the tensioner pulley is a common problem that results in slack in the belt elsewhere in the system. Specifically, when the tension in the “slack span” rapidly increases as a result of engine operating conditions, the belt may lift the tensioner pulley thereby allowing the length of the belt in the “slack span” to rapidly decrease. As a result of this change, the length of the belt in another span, referred to here as the “transient slack span,” may rapidly increase resulting in a sudden drop in tension and the possibility of slippage.
Accordingly, a tensioner is desired that is capable of resisting or compensating for lifting of the tensioner pulley in the “slack span” during transient operating conditions of the engine.